The present invention relates to an improvement in the field of anodizing aluminum and more particularly, but not by way of limitation, to an environmentally acceptable improved method for anodizing aluminum for durable, corrosion resistant adhesive bonding and paint adhesion.
It is well known that aluminum alloys are susceptible to corrosion. For maximum corrosion resistance it is now almost universally accepted to anodize aluminum by using a sulfuric acid solution followed by a sealing operation utilizing a chromated solution or, less often, by sealing the aluminum workpiece in a bath of boiling distilled water. Such a process is defined by the U.S. government in Mil-A-8625, Type II. This type of anodizing process produces a brittle oxide that is approximately 0.001-0.0003 inches thick and having typical coating weights of about 1000 mg/ft.sup.2. At coating weights above the specified minimum Type II requirement of 600 mg/ft.sup.2, the anodized workpiece experiences a significant decrease in fatigue properties. For this reason, the aircraft industry has preferred chromic acid anodizing. The U.S. government defines the chromic anodizing process in the Mil-A-8625, Type I specification. The chromic acid anodizing process yields acceptable corrosion resistance with coating weights as low as 200 mg/ft.sup.2. Anodic coatings formed in a chromic acid solution also require a sealing operation for optimal corrosion protection.
Prior to the sealing step, aluminum oxides formed in sulfuric acid and chromic acid have pore diameters of about 150 and 250 angstroms respectively. The sealing operation causes the oxide to hydrate and chemically swell, thus substantially closing the exposed pore openings. About 15 percent hydration is typical for chromic acid anodic coatings. Sealing, thus, results in negligible surface porosity for Type I and Type II anodic coatings.
Unfortunately, chromium is a known carcinogen. The amount of chromium waste that industry can legally discharge into the air or water has been severely restricted. Such restrictions has brought about efforts by metal finishing operations to find acceptable nonchromated alternatives. This is particularly true for the aircraft industry, which relies on chromic acid anodizing for providing much of the primary corrosion protection for its products. For reasons mentioned previously, fatigue considerations prohibit Type II sulfuric acid anodizing as a suitable process substitution. However, process variations of time and voltage have been introduced to produce thin film sulfuric acid anodic coatings from standard Type II baths. The thickness of these oxide films are difficult to control since coating weights may vary widely between 300 and 600 mg/ft.sup.2 depending upon the particular alloy and its position in the acid bath. The inability of this method to reliably yield a consistent coating has called into question the long term fatigue and corrosion resistance of the anodized product.
In a method described in U.S. Pat. No. 4,894,127, a dilute solution of sulfuric and boric acids produces a consistent anodic coating acceptable to the corrosion protection requirements of Mil-A-8625 Type I coatings, i.e., chromic acid anodizing. The coating thicknesses and weights obtained are also comparable to Type I coatings while pore diameters before sealing have been observed to be less than 100 angstroms. The percent of hydration due to sealing of about 5 percent indicates a very dense oxide. The addition of boric acid to the sulfuric acid electrolyte and/or the relatively high density of this coating may be the basis of fatigue and corrosion resistance of such anodized aluminum workpieces. Thus, sulfuric acid-boric acid anodizing is becoming increasingly favored by manufacturers of aircraft structures. This is true in spite of the prior shop practice of limiting the inevitable accumulation of dissolved aluminum in this solution to 3.7 grams per liter.
Although sulfuric acid-boric acid anodizing or thin film sulfuric acid anodizing may be environmentally acceptable alternatives to chromic acid anodizing, sealing in a chromated solution has until recently been viewed as a required practice. To eliminate chromium bearing solutions from all aspects of their anodizing processes, aircraft manufacturers have attempted to physically seal these oxides with polymers such as organic resins, or to coat the anodized aluminum with a suitable paint or a primer.
Resin sealing baths have been difficult and costly to maintain, generate undesirable quantities of waste and efforts to produce a reproducible uniform coating have been generally unsuccessful. Corrosion protection provided by pains and primers to unsealed anodic coatings rely upon good adhesion at the polymer/oxide interface. Problems of sporadic paint failures from sulfuric acid-boric acid and sulfuric acid anodic coatings have been experienced. Microscopic roughness related to oxide porosity is an apparent factor in determining the interlocking bonds desired to prevent these failures. The surface pore diameter must be of sufficient size to allow primer ingress and permeation into the oxide. Porosity of the oxide formed by chromic acid anodizing provides a level of adhesion performance not realized with nonchromated anodizing processes.
Much more stringent requirements for long term bond stability under stressed, hot, wet, and corrosive conditions distinguish the qualities required for bonded aircraft structures as opposed to those for paint adhesion. Mil-A-8625 Type I, Type II, sulfuric/boric acid, and thin film sulfuric acid anodizing are generally precluded from use as aluminum surface treatments prior to structural adhesive bonding. Occasional exceptions have been made in the case of aluminum bonded to galvanically dissimilar metals such as titanium and steel.
For aluminum to aluminum adhesive bonding, the prior practice has been to pickle the adherends in an elevated temperature FPL etch solution, i.e. a sodium dichromate and sulfuric acid solution. Aluminum surfaces treated in this manner produced very thin oxides of about 200 angstroms in thickness with open cells. This method produced erratic adhesive bond results until it was learned how to optimize the solution prior to use.
The following patents, while of interest in the general field to which the invention pertains, do not disclose the particular aspects of the present invention that are of significant interest.
U.S. Pat. No. 4,085,012 describes a phosphoric acid anodizing process for preparing aluminum for adhesive bonding. The oxide produced on the aluminum is between 4000 and 7000 angstroms thick depending upon the particular aluminum alloy and has anodic coating weights of about 35 and 80 mg/ft.sup.2 for 2024 bare aluminum and 2024 clad aluminum respectively. The pores of this oxide are thin walled and have large pores about 400 angstrom in diameter, i.e. four times greater than what is produced by sulfuric/boric acid anodizing. Although this anodic coating is thin, fragile and provides little intrinsic corrosion inhibiting properties, the process produces environmentally stable aluminum surfaces for adhesive bonding. This thin oxide is not a fatigue concern and the superior adhesion to epoxy primers provide adequate corrosion protection for aluminum to aluminum bonded assemblies. Thus, phosphoric acid anodize has displaced the FPL etch for most aircraft bonding applications.
U.S. Pat. No. 3,940,321 discloses a method of anodizing an aluminum lithographic plate which comprises the steps of firstly anodizing the aluminum by electrolysis in sulfuric acid solution, and secondly, anodizing the aluminum by electrolysis in a phosphoric acid solution. The surface of the aluminum is preferably grained first, by electrolysis in dilute hydrochloric acid. It has been found that the method of this patent yields uncertain results since the phosphoric acid is very aggressive on the anodized region that has been anodized by the sulfuric acid solution.
As previously mentioned, thicker anodic coatings are used for adhesive bonding when dissimilar metals are fayed together. For these bond applications, the thin and delicate FPL etch and phosphoric acid anodize coatings do not provide the insulative separation desired to prevent galvanic coupling. Also, additional protection of the aluminum is beneficial for corrosion prone, dissimilar metal bond applications. Prior art for this unique bonding application has been to use aluminum treated per the Mil-A -8625, Type I specification. Considerations of fatigue resistance, dielectric separation, and inherent corrosion resistance are reasons for preferring chromic acid anodizing. Although unsealed Type I oxide coatings have the largest surface porosity and provides the best adhesion of any industrially practiced corrosion inhibiting anodize treatment, it is deficient in its wet peel strength with epoxy resins when compared to oxides formed in phosphoric acid. Thus, problems with less than optimum bond strength and chromium effluents remain with manufacturers of aircraft components requiring adhesive bonding of dissimilar metals.